By Jonathan Marcus
BBC diplomatic correspondent
The UK and US have held high level talks on the possibility of putting ballistic missile interceptors on British soil. The location of facilities continues to stir debate.
Protests have taken place in the Czech Republic over a radar system
Its advocates believe that the deployment of anti-missile defences is an essential response to the spread of ever-longer range ballistic missiles to a variety of unstable regimes.
The system's critics charge that the whole idea is simply "a Maginot Line" in the sky; a reference to the cumbersome system of forts and fixed defences that were skirted by German troops at the start of World War II.
So who is right?
There is certainly no doubt that countries like North Korea and Iran are developing longer-range missiles capable of carrying chemical or even maybe nuclear warheads.
The Nuclear Non-Proliferation Treaty - the international agreement intended to halt the spread of nuclear weapons - is under strain like never before.
The world is getting more dangerous. And under this reading missile defences to counter an attack by a so-called "rogue state" could make obvious sense.
The United States feels especially vulnerable.
And it is no accident that the enthusiasm for missile defence grew markedly in the wake of the 9/11 attacks.
Of course the 'weapons' of choice in 9/11 were not ballistic missiles but fuel-laden airliners.
Nonetheless, the advocates of defensive systems say that missile technology is spreading at an alarming rate.
The critics of the US missile defence system do have one killer point in their favour.
For now, at least, it appears far from certain that the prototype system - 14 interceptor missiles deployed at Fort Greely in Alaska, with two more at Vandenberg Air Force Base in California - actually works.
It has not yet been declared fully operational by the Americans; test results have been mixed to say the least; and it is fundamentally a work in progress.
Work in progress or not it is already prompting tensions with the Russian government up in arms and anti-nuclear protestors in Europe dusting off their placards and gaining renewed vigour.
The bone of contention is the need to locate certain parts of the system in Europe.
This is to defend the US against missiles potentially coming from the Middle East.
A prototype system is located at the Vandenberg Air Force Base
The early warning radars at Fylingdales in Britain have been upgraded to act as part of the global network of land and space-based sensors that are the eyes and ears of the system.
An additional radar could well be built in the Czech Republic.
The crux of the issue though is where additional interceptor missiles should be based.
One option is Poland; indeed the Polish Government is eager to host them on its soil.
And it is now clear that the British Government too is exploring the options with Washington.
That could revive the British peace movement which fought a strong, though unsuccessful, battle to prevent the deployment of new intermediate-range US nuclear missiles in Europe during the 1980s.
It might also exacerbate tensions within Nato on how far its European members should go in hosting facilities intended largely for US defence.
It is politics that is largely driving the Czech and Polish enthusiasm to host parts of the system.
They would both underpin their relationship with Washington and underline yet again that they are no longer in the Russian orbit.
Nonetheless domestic political instability in central Europe could give Britain the edge as a favoured site, again on political grounds.
All of this is prompting some tough words from Moscow.
The Russians insist on seeing new missile defences as in some way intended to weaken their own nuclear arsenal.
Russian leaders and generals warn they may develop new weapons to get around the US system.
They are also directing especially caustic remarks at Poland and the Czech Republic with explicit threats that Russia might again deploy medium-range nuclear missiles presumably targeted against Europe.
'Son of Star Wars'
All of this is despite the obvious limitations of the existing system both in capability and scale.
The Americans insist that the Russians understand full well that it is not directed at them.
But that is not how Moscow is choosing to see it.
Some pundits have dubbed the new US system the "son of Star Wars".
The reference to the Reagan-era defensive shield that aspired to make nuclear weapons obsolete represents a wild exaggeration.
But the label is sticking.
And there is a danger that "son of Star Wars" could provoke something that looks a little like a reprise of the Cold War with Moscow.